Network Working Group R. Gieben
Internet-Draft Google
Intended status: Informational W. Mekking
Expires: August 7, 2014 NLnet Labs
February 3, 2014
Authenticated Denial of Existence in the DNSdraft-gieben-auth-denial-of-existence-dns-06
Abstract
Authenticated denial of existence allows a resolver to validate that
a certain domain name does not exist. It is also used to signal that
a domain name exists, but does not have the specific RR type you were
asking for. When returning a negative DNSSEC response, a name server
usually includes up to two NSEC records. With NSEC3 this amount is
three.
This document provides additional background commentary and some
context for the NSEC and NSEC3 mechanisms used by DNSSEC to provide
authenticated denial of existence responses
Status of This Memo
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This Internet-Draft will expire on August 7, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Denial of existence is a mechanism that informs a resolver that a
certain domain name does not exist. It is also used to signal that a
domain name exists, but does not have the specific RR type you were
asking for.
The first is referred to as an NXDOMAIN (non-existent domain)
([RFC2308] Section 2.1) and the latter a NODATA ([RFC2308]
Section 2.2) response. Both are also known as negative responses.
Authenticated denial of existence uses cryptography to sign the
negative response. However, if there is no answer, what is it that
needs to be signed? To further complicate this matter, there is the
desire to pre-generate negative responses that are applicable for all
queries for non-existent names in the signed zone. See Section 3 for
the details.
In this document, we will explain how authenticated denial of
existence works. We begin by explaining the current technique in the
DNS and work our way up to DNSSEC. We explain the first steps taken
in DNSSEC and describe how NSEC and NSEC3 work. The NXT, NO, NSEC2
and DNSNR records also briefly make their appearance, as they have
paved the way for NSEC and NSEC3.
To complete the picture, we also need to explain DNS wildcards as
these complicate matters, especially combined with CNAME records.
Note: In this document, domain names in zone file examples will have
a trailing dot, in the running text they will not. This text is
written for people who have a fair understanding of DNSSEC. The
following RFCs are not required reading, but they help in
understanding the problem space.
o RFC 5155 [RFC5155] - Hashed Authenticated Denial of Existence;
o RFC 4592 [RFC4592] - The Role of Wildcards in the DNS.
And these provide some general DNSSEC information.
o RFC 4033, RFC 4034, RFC 4035 [RFC4033], [RFC4034], [RFC4035] -
DNSSEC Specification;
o RFC 4956 [RFC4956] - DNS Security (DNSSEC) Opt-In. This RFC has
experimental status, but is a good read.
These three drafts give some background information on the NSEC3
development.
o The NO record [I-D.ietf-dnsext-not-existing-rr];
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o The NSEC2 record [I-D.laurie-dnsext-nsec2v2];
o The DNSNR record [I-D.arends-dnsnr].
2. Denial of Existence
We start with the basics and take a look at NXDOMAIN handling in the
DNS. To make it more visible we are going to use a small DNS zone,
with 3 names ("example.org", "a.example.org" and "d.example.org") and
3 types (SOA, A and TXT). For brevity, the class is not shown
(defaults to IN) and the SOA record is shortened, resulting in the
following zone file:
example.org. SOA ( ... )
example.org. NS a.example.org.
a.example.org. A 192.0.2.1
TXT "a record"
d.example.org. A 192.0.2.1
TXT "d record"
Figure 1: The unsigned "example.org" zone.
2.1. NXDOMAIN Responses
If a resolver asks for the TXT type belonging to "a.example.org" to
the name server serving this zone, it sends the following question:
"a.example.org TXT"
The name server looks in its zone data and generates an answer. In
this case a positive one: "Yes it exists and this is the data",
resulting in this reply:
;; status: NOERROR, id: 28203
;; ANSWER SECTION:
a.example.org. TXT "a record"
;; AUTHORITY SECTION:
example.org. NS a.example.org.
The "status: NOERROR" signals that everything is OK, "id" is an
integer used to match questions and answers. In the ANSWER section,
we find our answer. The AUTHORITY section holds the names of the
name servers that have information concerning the "example.org" zone.
Note that including this information is optional.
If a resolver asks for "b.example.org TXT" it gets an answer that
this name does not exist:
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;; status: NXDOMAIN, id: 7042
;; AUTHORITY SECTION:
example.org. SOA ( ... )
In this case, we do not get an ANSWER section and the status is set
to NXDOMAIN. From this the resolver concludes that "b.example.org"
does not exist. The AUTHORITY section holds the SOA record of
"example.org" that the resolver can use to cache the negative
response.
2.2. NODATA Responses
It is important to realize that NXDOMAIN is not the only type of
does-not-exist. A name may exist, but the type you are asking for
may not. This occurrence of non-existence is called a NODATA
response. Let us ask our name server for "a.example.org AAAA", and
look at the answer:
;; status: NOERROR, id: 7944
;; AUTHORITY SECTION:
example.org. SOA ( ... )
The status NOERROR shows that the "a.example.org" name exists, but
the reply does not contain an ANSWER section. This differentiates a
NODATA response from an NXDOMAIN response, the rest of the packet is
very similar. The resolver has to put these pieces of information
together and conclude that "a.example.org" exists, but it does not
have an "AAAA" record.
3. Secure Denial of Existence
The above has to be translated to the security aware world of DNSSEC.
But there are a few principles DNSSEC brings to the table:
1. A name server is free to compute the answer and signature(s) on-
the-fly, but the protocol is written with a "first sign, then
load" attitude in mind. It is rather asymmetrical, but a lot of
the design in DNSSEC stems from fact that you need to accommodate
authenticated denial of existence. If the DNS did not have
NXDOMAIN, DNSSEC would be a lot simpler, but a lot less useful!
2. The DNS packet header is not signed. This means that a "status:
NXDOMAIN" can not be trusted. In fact the entire header may be
forged, including the AD bit (AD stands for Authentic Data, see
RFC 3655 [RFC3655]), which may give some food for thought;
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3. DNS wildcards and CNAME records complicate matters significantly.
More about this in later sections (Section 5.3 and Section 5.4).
The first principle implies that all denial of existence answers need
to be pre-computed, but it is impossible to pre-compute (all
conceivable) non-existence answers.
A generic denial record which can be used in all denial of existence
proofs is not an option: such a record is susceptible to replay
attacks. When you are querying a name server for any record that
actually exists, a man-in-the-middle could replay that generic denial
record that is unlimited in its scope and it would be impossible to
tell whether the response was genuine or spoofed. In other words,
the generic record can be replayed to falsely deny _all_ possible
responses.
We could also use the QNAME in the answer and sign that; essentially
signing an NXDOMAIN response. While this approach could have worked
technically, it is incompatible with off-line signing.
The way this has been solved is by introducing a record that defines
an interval between two existing names. Or to put it another way: it
defines the holes (non-existing names) in the zone. This record can
be signed beforehand and given to the resolver. Appendix A and
Appendix B describe on-line signing techniques that are compatible
with this scheme.
Given all these troubles, why didn't the designers of DNSSEC go
for the (easy) route and allowed for on-line signing? Well, at
that time (pre 2000), on-line signing was not feasible with the
then current hardware. Keep in mind that the larger servers get
between 2000 and 6000 queries per second (qps), with peaks up to
20,000 qps or more. Scaling signature generation to these kind of
levels is always a challenge. Another issue was (and is) key
management, for on-line signing to work _each_ authoritative name
server needs access to the private key(s). This is considered a
security risk. Hence, the protocol required not to rely on on-
line signing.
The road to the current solution (NSEC/NSEC3) was long. It started
with the NXT (next) record. The NO (not existing) record was
introduced, but never made it to RFC. Later on, NXT was superseded
by the NSEC (next secure) record. From there it went through NSEC2/
DNSNR to finally reach NSEC3 (next secure, version 3) in RFC 5155.
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Internet-Draft Authenticated Denial in DNS February 20143.1. NXT
The first attempt to specify authenticated denial of existence was
NXT (RFC 2535 [RFC2535]). Section 5.1 of that RFC introduces the
record:
"The NXT resource record is used to securely indicate that RRs
with an owner name in a certain name interval do not exist in a
zone and to indicate what RR types are present for an existing
name."
By specifying what you do have, you implicitly tell what you don't
have. NXT is superseded by NSEC. In the next section we explain how
NSEC (and thus NXT) works.
3.2. NSEC
In RFC 3755 [RFC3755] all the DNSSEC types were given new names, SIG
was renamed RRSIG, KEY became DNSKEY and NXT was renamed to NSEC and
a minor issue was fixed in the process, namely the type bitmap was
redefined to allow more than 127 types to be listed ([RFC2535],
Section 5.2).
Just as NXT, NSEC is used to describe an interval between names: it
indirectly tells a resolver which names do not exist in a zone.
For this to work, we need our "example.org" zone to be sorted in
canonical order ([RFC4034], Section 6.1), and then create the NSECs.
We add three NSEC records, one for each name, and each one covers a
certain interval. The last NSEC record points back to the first as
required by the RFC, and depicted in Figure 2.
1. The first NSEC covers the interval between "example.org" and
"a.example.org";
2. The second NSEC covers "a.example.org" to "d.example.org";
3. The third NSEC points back to "example.org", and covers
"d.example.org" to "example.org" (i.e. the end of the zone).
As we have defined the intervals and put those in resource records,
we now have something that can be signed.
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does precisely that: "b" should come after "a", but the next owner
name is "d.example.org", so "b" does not exist.
Only by making that calculation, is a resolver able to conclude that
the name "b" does not exist. If the signature of the NSEC record is
valid, "b" is proven not to exist. We have authenticated denial of
existence.
Note that a man-in-the-middle may still replay this NXDOMAIN response
when you're querying for, say, "c.example.org". But it would not do
any harm since it is provably the proper response to the query.
3.3. NODATA Responses
NSEC records are also used in NODATA responses. In that case we need
to look more closely at the type bitmap. The type bitmap in an NSEC
record tells which types are defined for a name. If we look at the
NSEC record of "a.example.org", we see the following types in the
bitmap: A, TXT, NSEC and RRSIG. So for the name "a" this indicates
we must have an A, TXT, NSEC and RRSIG record in the zone.
With the type bitmap of the NSEC record, a resolver can establish
that a name is there, but the type is not. For example, if a
resolver asks for "a.example.org AAAA", the reply that comes back is:
;; status: NOERROR, id: 44638
;; AUTHORITY SECTION:
example.org. SOA ( ... )
example.org. RRSIG(SOA) ( ... )
a.example.org. NSEC d.example.org. A TXT RRSIG NSEC
a.example.org. RRSIG(NSEC) ( ... )
The resolver should check the AUTHORITY section and conclude that:
(1) "a.example.org" exists (because of the NSEC with that owner
name) and;
(2) the type (AAAA) does not as it is not listed in the type bitmap.
The techniques used by NSEC form the basics of authenticated denial
of existence in DNSSEC.
3.4. Drawbacks of NSEC
There were two issues with NSEC (and NXT). The first is that it
allows for zone walking. NSEC records point from one name to
another, in our example: "example.org", points to "a.example.org"
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which points to "d.example.org" which points back to "example.org".
So we can reconstruct the entire "example.org" zone, thus defeating
attempts to administratively block zone transfers ([RFC2065]
Section 5.5).
The second issue is that when a large, delegation-centric ([RFC5155],
Section 1.1), zone deploys DNSSEC, every name in the zone gets an
NSEC plus RRSIG. So this leads to a huge increase in the zone size
(when signed). This would in turn mean that operators of such zones
who are deploying DNSSEC, face up front costs. This could hinder
DNSSEC adoption.
These two issues eventually lead to NSEC3 which:
o Adds a way to garble the owner names, thus thwarting zone walking;
o Makes it possible to skip names for the next owner name. This
feature is called Opt-Out (See Section 5.1). It means not all
names in your zone get an NSEC3 plus ditto signature, making it
possible to "grow into" your DNSSEC deployment.
Note that there are other ways to mitigate against zone walking. RFC4470 [(#RFC4470) prevents zone walking by introducing minimally
covering NSEC records. This technique is described in Appendix A.
Before we delve into NSEC3, let us first take a look at its
predecessors: NO, NSEC2, and DNSNR.
4. Experimental and Deprecated Mechanisms: NO, NSEC2 and DNSNR
Long before NSEC was defined, the NO record was introduced. It was
the first record to use the idea of hashed owner names, to fix the
issue of zone walking that was present with the NXT record. It also
fixed the type bitmap issue of the NXT record, but not in a space
efficient way. At that time (around 2000) zone walking was not
considered important enough to warrant the new record. People were
also worried that DNSSEC deployment would be hindered by developing
an alternate means of denial of existence. Thus the effort was
shelved and NXT remained.
When the new DNSSEC specification [RFC4034] was written, people were
still not convinced that zone walking was a problem that should be
solved. So NSEC saw the light and inherited the two issues from NXT.
Several years after, NSEC2 was introduced as a way to solve the two
issues of NSEC. The NSEC2 draft contains the following paragraph:
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"This document proposes an alternate scheme which hides owner
names while permitting authenticated denial of existence of non-
existent names. The scheme uses two new RR types: NSEC2 and
EXIST."
When an authenticated denial of existence scheme starts to talk about
EXIST records, it is worth paying extra attention. The EXIST record
was defined as a record without RDATA that would be used to signal
the presence of a domain name. From the draft:
"In order to prove the nonexistence of a record that might be
covered by a wildcard, it is necessary to prove the existence of
its closest encloser. This record does that. Its owner is the
closest encloser. It has no RDATA. If there is another RR that
proves the existence of the closest encloser, this SHOULD be used
instead of an EXIST record."
The introduction of this record led to questions on what wildcards
actually mean (especially in the context of DNSSEC). It is probably
not a coincidence that "The Role of Wildcards in the Domain Name
System" ([RFC4592]) was standardized before NSEC3 was.
NSEC2 solved the zone walking issue by hashing (with SHA1 and a salt)
the "next owner name" in the record, thereby making it useless for
zone walking. But it did not have Opt-Out.
The DNSNR record was another attempt that used hashed names to foil
zone walking and it also introduced the concept of opting out (called
"Authoritative Only Flag") which limited the use of DNSNR in
delegation-centric zones.
All these proposals didn't make it, but did provide valuable
insights. To summarize:
o The NO record introduced hashing, but this idea lingered in the
background for a long time;
o The NSEC2 record made it clear that wildcards were not completely
understood;
o The DNSNR record used a new flag field in the RDATA to signal Opt-
Out;
5. NSEC3
From the experience gained with NSEC2 and DNSNR, NSEC3 was forged.
It incorporates both Opt-Out and the hashing of names. NSEC3 solves
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any issues people might have with NSEC, but it introduces some
additional complexity.
NSEC3 did not supersede NSEC, they are both defined for DNSSEC. So
DNSSEC is blessed with two different means to perform authenticated
denial of existence: NSEC and NSEC3. In NSEC3 every name is hashed,
including the owner name. This means that NSEC3 chain is sorted in
hash order, instead of canonical order. Because the owner names are
hashed, the next owner name for "example.org" is unlikely to be
"a.example.org". Because the next owner name is hashed, zone walking
becomes more difficult.
To make it even more difficult to retrieve the original names, the
hashing can be repeated several times each time taking the previous
hash as input. To prevent the reuse of pre-generated hash values
between zones a (per zone) salt can also be added. In the NSEC3 for
"example.org" we have hashed the names thrice ([RFC5155], Section 5)
and use the salt "DEAD". Lets look at typical NSEC3 record:
15bg9l6359f5ch23e34ddua6n1rihl9h.example.org. (
NSEC3 1 0 2 DEAD A6EDKB6V8VL5OL8JNQQLT74QMJ7HEB84
NS SOA RRSIG DNSKEY NSEC3PARAM )
On the first line we see the hashed owner name:
"15bg9l6359f5ch23e34ddua6n1rihl9h.example.org", this is the hashed
name of the fully qualified domain name (FQDN) "example.org" encoded
as Base32 ([RFC4648]). Note that even though we hashed
"example.org", the zone's name is added to make it look like a domain
name again. In our zone, the basic format is
"Base32(SHA1(FQDN)).example.org". The next hashed owner name
"A6EDKB6V8VL5OL8JNQQLT74QMJ7HEB84" (line 2) is the hashed version of
"d.example.org", represented as Base32. Note that "d.example.org" is
used are the next owner name, because in the hash ordering, its hash
comes after the hash of the zone's apex. Also note that
".example.org" is not added to the next hashed owner name, as this
name always falls in the current zone.
The "1 0 2 DEAD" section of the NSEC3 states:
o Hash Algorithm = 1 (SHA1, this is the default, no other hash
algorithms are currently defined for use in NSEC3);
o Opt-Out = 0 (disabled);
o Hash Iterations = 2, this yields three iterations, as a zero value
is already one iteration;
o Salt = "DEAD".
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At the end we see the type bitmap, which is identical to NSEC's
bitmap, that lists the types present at the original owner name.
Note that the type NSEC3 is absent from the list in the example
above. This is due to the fact that the original owner name
("example.org") does not have the NSEC3 type. It only exists for the
hashed name.
Names like "1.h.example.org" hash to one label in NSEC3,
"1.h.example.org" becomes:
"117gercprcjgg8j04ev1ndrk8d1jt14k.example.org" when used as an owner
name. This is an important observation. By hashing the names you
lose the depth of a zone - hashing introduces a flat space of names,
as opposed to NSEC.
The name used above ("1.h.example.org") creates an empty non-
terminal. Empty non-terminals are domain names that have no RRs
associated with them, and exist only because they have one or more
sub-domains that do ([RFC5155], Section 1.3). The record:
1.h.example.org. TXT "1.h record"
creates two names:
1. "1.h.example.org" that has the type: TXT;
2. "h.example.org" which has no types. This is the empty non-
terminal.
An empty non-terminal will get an NSEC3 record, but not an NSEC
record. In Section 5.5 is shown how the resolver uses these NSEC3
records to validate the denial of existence proofs.
Note that NSEC3 might not always be useful. For example, highly
structures zones, like the reverse zones ip6.arpa and in-addr.arpa,
can be walked even with NSEC3 due to their structure. Also the names
in small, trivial zones can be easily guessed. In these cases, it
does not help to defend against zone walking, but does add the
computational load on authoritative servers and validators.
5.1. Opt-Out
Hashing mitigates the zone walking issue of NSEC. The other issue,
the high costs of securing a delegation to an insecure zone, is
tackled with Opt-Out. When using Opt-Out, names that are an insecure
delegation (and empty non-terminals that are only derived from
insecure delegations) don't require an NSEC3 record. For each
insecure delegation, the zone size can be decreased (compared with a
fully signed zone without using Opt-Out) with at least two records:
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one NSEC3 record and one corresponding RRSIG record. If the insecure
delegation would introduce empty non-terminals, even more records can
be omitted from the zone.
Opt-Out NSEC3 records are not able to prove or deny the existence of
the insecure delegations. In other words, those delegation do not
benefit from the cryptographic security that DNSSEC provides.
A recently discovered corner case ([RFC5155-errata3441]) shows that
not only those delegations remain insecure, also the empty non-
terminal space that is derived from those delegations are insecure.
Because the names in this empty non-terminal space do exist according
to the definition in [RFC4592], the server should respond to queries
for these names with a NODATA response. However, the validator
requires an NSEC3 record proving the NODATA response ([RFC5155],
Section 8.5):
"The validator MUST verify that an NSEC3 RR that matches QNAME is
present and that both the QTYPE and the CNAME type are not set in
its Type Bit Maps field."
A way to resolve this contradiction in the specification is to always
provide empty non-terminals with an NSEC3 record, even if it is only
derived from an insecure delegation.
5.2. Loading an NSEC3 Zone
Whenever an authoritative server receives a query for a non-existing
record, it has to hash the incoming query name to determine into
which interval between two existing hashes it falls. To do that it
needs to know the zone's specific NSEC3 parameters (hash iterations
and salt).
One way to learn them is to scan the zone during loading for NSEC3
records and glean the NSEC3 parameters from them. However, it would
need to make sure that there is at least one complete set of NSEC3
records for the zone using the same parameters. Therefore, it would
need to inspect all NSEC3 records.
A more graceful solution was designed. The solution was to create a
new record, NSEC3PARAM, which must be placed at the apex of the zone.
Its role is to provide a fixed place where an authoritative name
server can directly see the NSEC3 parameters used, and by putting it
in the zone it allows for easy transfer to the secondaries. If NSEC3
were designed in the early days of DNS (+/- 1984) this information
would probably have been put in the SOA record.
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Internet-Draft Authenticated Denial in DNS February 20145.3. Wildcards in the DNS
So far, we have only talked about denial of existence in negative
responses. However, denial of existence may also occur in positive
responses, i.e., where the ANSWER section of the response is not
empty. This can happen because of wildcards.
Wildcards have been part of the DNS since the first DNS RFCs. They
allow to define all names for a certain type in one go. In our
"example.org" zone we could for instance add a wildcard record:
*.example.org. TXT "wildcard record"
For completeness, our (unsigned) zone now looks like this:
example.org. SOA ( ... )
example.org. NS a.example.org.
*.example.org. TXT "wildcard record"
a.example.org. A 192.0.2.1
TXT "a record"
d.example.org. A 192.0.2.1
TXT "d record"
Figure 4: The example.org zone with a wildcard record.
If a resolver asks for "z.example.org TXT", the name server will
respond with an expanded wildcard, instead of an NXDOMAIN:
;; status: NOERROR, id: 13658
;; ANSWER SECTION:
z.example.org. TXT "wildcard record"
Note however that the resolver can not detect that this answer came
from a wildcard. It just sees the answer as-is. How will this
answer look with DNSSEC?
;; status: NOERROR, id: 51790
;; ANSWER SECTION:
z.example.org. TXT "wildcard record"
z.example.org. RRSIG(TXT) ( ... )
;; AUTHORITY SECTION:
d.example.org. NSEC example.org. A TXT RRSIG NSEC
d.example.org. RRSIG(NSEC) ( ... )
Figure 5: A response with an expanded wildcard and with DNSSEC.
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The RRSIG of the "z.example.org" TXT record indicates there is a
wildcard configured. The RDATA of the signature lists a label count
[RFC4034], Section 3.1.3., of two (not visible in the answer above),
but the owner name of the signature has three labels. This mismatch
indicates there is a wildcard "*.example.org" configured.
An astute reader may notice that it appears as if a
"z.example.org" RRSIG(TXT) is created out of thin air. This is
not the case. The signature for "z.example.org" does not exist.
The signature you are seeing is the one for "*.example.org" which
does exist, only the owner name is switched to "z.example.org".
So even with wildcards, no signatures have to be created on the
fly.
The DNSSEC standard mandates that an NSEC (or NSEC3) is included in
such responses. If it wasn't, an attacker could mount a replay
attack and poison the cache with false data: Suppose that the
resolver has asked for "a.example.org TXT". An attacker could modify
the packet in such way that it looks like the response was generated
through wildcard expansion, even though there exists a record for
"a.example.org TXT".
The tweaking simply consists of adjusting the ANSWER section to:
;; status: NOERROR, id: 31827
;; ANSWER SECTION:
a.example.org. TXT "wildcard record"
a.example.org. RRSIG(TXT) ( ... )
Figure 6: A forged response without the expanded wildcard.
Note the subtle difference from Figure 5 in the owner name. In this
response we see a "a.example.org TXT" record, for which a record with
different RDATA (See Figure 4) exist in the zone.
Which would be a perfectly valid answer if we would not require the
inclusion of an NSEC or NSEC3 record in the wildcard answer response.
The resolver believes that "a.example.org TXT" is a wildcard record,
and the real record is obscured. This is bad and defeats all the
security DNSSEC can deliver. Because of this, the NSEC or NSEC3 must
be present.
Another way of putting this is that DNSSEC is there to ensure the
name server has followed the steps as outlined in [RFC1034],
Section 4.3.2 for looking up names in the zone. It explicitly lists
wildcard look up as one of these steps (3c), so with DNSSEC this must
be communicated to the resolver: hence the NSEC(3) record.
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The NSEC record "*.a.example.org" proves that wildcard expansion to
"w.a.example.org" was appropriate: "w.a." falls in the gap "*.a" to
"*.b". Similar, the NSEC record "*.b.example.org" proves that there
was no direct match for "w.b.example.org" and "*.c.example.org"
denies the direct match for "w.c.example.org".
DNAME records and wildcard names should not be used as reiterated in
[RFC6672] Section 3.3.
5.5. The Closest Encloser NSEC3 Record
We can have one or more NSEC3 records that deny the existence of the
requested name and one NSEC3 record that deny wildcard synthesis.
What do we miss?
The short answer is that, due to the hashing in NSEC3 you loose the
depth of your zone: Everything is hashed into a flat plane. To make
up for this loss of information you need an extra record.
To understand NSEC3, we will need two definitions:
Closest encloser: Introduced in [RFC4592], "The closest encloser is
the node in the zone's tree of existing domain names that has the
most labels matching the query name (consecutively, counting from
the root label downward)." In our example, if the query name is
"x.2.example.org" then "example.org" is the "closest encloser";
Next closer name: Introduced in the NSEC3 RFC, this is the closest
encloser with one more label added to the left. So if
"example.org" is the closest encloser for the query name
"x.2.example.org", "2.example.org" is the "next closer name".
An NSEC3 "closest encloser proof" consists of:
1. An NSEC3 record that *matches* the "closest encloser". This
means the unhashed owner name of the record is the closest
encloser. This bit of information tells a resolver: "The name
you are asking for does not exist, the closest I have is this".
2. An NSEC3 record that *covers* the "next closer name". This means
it defines an interval in which the "next closer name" falls.
This tells the resolver: "The next closer name falls in this
interval, and therefore the name in your question does not exist.
In fact, the closest encloser is indeed the closest I have".
These two records already deny the existence of the requested name,
so we do not need an NSEC3 record that covers the actual queried
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name: By denying the existence of the next closer name, you also deny
the existence of the queried name.
Note that with NSEC, the existence of all empty non-terminals between
the two names are denied, hence implicitly contains the closest
encloser.
For a given query name, there is one (and only one) place where
wildcard expansion is possible. This is the "source of synthesis",
and is defined ([RFC4592], Section 2.1.1 and Section 3.3.1) as:
<asterisk label>.<closest encloser>
In other words, to deny wildcard synthesis, the resolver needs to
know the hash of the source of synthesis. Since it does not know
beforehand what the closest encloser of the query name is, it must be
provided in the answer.
Take the following example. We take our zone, and put two TXT
records to it. The records added are "1.h.example.org" and
"3.3.example.org". It is signed with NSEC3, resulting in the
following unsigned zone.
example.org. SOA ( ... )
example.org. NS a.example.org.
1.h.example.org. TXT "1.h record"
3.3.example.org. TXT "3.3 record"
Figure 8: The added TXT records in example.org. These records create
two empty non-terminals: h.example.org and 3.example.org.
The resolver asks the following: "x.2.example.org TXT". This leads
to an NXDOMAIN response from the server, which contains three NSEC3
records. A list of hashed owner names can be found in Appendix C.
Also see Figure 9 the numbers in that figure correspond with the
following NSEC3 records:
15bg9l6359f5ch23e34ddua6n1rihl9h.example.org. (
NSEC3 1 0 2 DEAD 1AVVQN74SG75UKFVF25DGCETHGQ638EK NS SOA RRSIG
DNSKEY NSEC3PARAM )
1avvqn74sg75ukfvf25dgcethgq638ek.example.org. (
NSEC3 1 0 2 DEAD 75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ )
75b9id679qqov6ldfhd8ocshsssb6jvq.example.org. (
NSEC3 1 0 2 DEAD 8555T7QEGAU7PJTKSNBCHG4TD2M0JNPJ TXT RRSIG )
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If we would follow the NSEC approach, the resolver is only interested
in one thing. Does the hash of "x.2.example.org" fall in any of the
intervals of the NSEC3 records it got?
example.org
**
+-- ** . . . . . . . . . . .
(1) / . ^ . .
/ . | . .
| . | . .
v . | . .
** | (2) ** ++
h.example.org ** ----+----> ** 3.example.org ++ 2.example.org
. / . | .
. / (5) . | (3) .
. / . | .
. / . v .
1.h.example.org ** ** ++
** <--------- ** 3.3.example.org ++ x.2.example.org
(4)
Figure 9: x.2.example.org does not exist. The five arrows represent
the NSEC3 records, the ones numbered (1), (2) and (3) are the NSEC3s
returned in our answer. 2.example.org is covered by (3) and
x.2.example.org is covered by (4).
The hash of "x.2.example.org" is "ndtu6dste50pr4a1f2qvr1v31g00i2i1".
Checking this hash on the first NSEC3 yields that it does not fall in
between the interval: "15bg9l6359f5ch23e34ddua6n1rihl9h" and
"1avvqn74sg75ukfvf25dgcethgq638ek". For the second NSEC3 the answer
is also negative: the hash sorts outside the interval described by
"1avvqn74sg75ukfvf25dgcethgq638ek" and
"75b9id679qqov6ldfhd8ocshsssb6jvq". And the third NSEC3, with
interval "75b9id679qqov6ldfhd8ocshsssb6jvq" to
"8555t7qegau7pjtksnbchg4td2m0jnpj" also isn't of any help.
What is a resolver to do? It has been given the maximum amount of
NSEC3s and they all seem useless.
So this is where the closest encloser proof comes into play. And for
the proof to work, the resolver needs to know what the closest
encloser is. There must be an existing ancestor in the zone: a name
must exist that is shorter than the query name. The resolver keeps
hashing increasingly shorter names from the query name until an owner
name of an NSEC3 matches. This owner name is the closest encloser.
When the resolver has found the closest encloser, the next step is to
construct the next closer name. This is the closest encloser with
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the last chopped label from query name pre-pended to it: "<last
chopped label>.<closest encloser>". The hash of this name should be
covered by the interval set in any of the NSEC3 records.
Then the resolver needs to check the presence of a wildcard. It
creates the wildcard name by pre-pending the asterisk label to the
closest encloser: "*.<closest encloser>", and uses the hash of that.
Going back to our example, the resolver must first detect the NSEC3
that matches the closest encloser. It does this by chopping up the
query name, hashing each instance (with the same number of iterations
and hash as the zone it is querying) and comparing that to the
answers given. So it has the following hashes to work with:
x.2.example.org: "ndtu6dste50pr4a1f2qvr1v31g00i2i1", last chopped
label: "<empty>";
2.example.org: "7t70drg4ekc28v93q7gnbleopa7vlp6q", last chopped
label: "x";
example.org: "15bg9l6359f5ch23e34ddua6n1rihl9h", last chopped label:
"2";
Of these hashes only one matches the owner name of one of the NSEC3
records: "15bg9l6359f5ch23e34ddua6n1rihl9h". This must be the
closest encloser (unhashed: "example.org"). That's the main purpose
of that NSEC3 record: tell the resolver what the closest encloser is.
When using Opt-Out, it is possible that the actual closest encloser
to the QNAME does not have an NSEC3 record. If so, we will have to
do with the closest provable encloser, which is the closest enclosing
authoritative name that does have a NSEC3 record. In the worst case,
this is the NSEC3 record corresponding to the apex, this name must
always have an NSEC3 record.
With the closest (provable) encloser, the resolver constructs the
next closer, which in this case is: "2.example.org"; "2" is the last
label chopped, when "example.org" is the closest encloser. The hash
of this name should be covered in any of the other NSEC3s. And it is,
"7t70drg4ekc28v93q7gnbleopa7vlp6q" falls in the interval set by:
"75b9id679qqov6ldfhd8ocshsssb6jvq" and
"8555t7qegau7pjtksnbchg4td2m0jnpj" (this is our second NSEC3).
So what does the resolver learn from this?
o "example.org" exists;
o "2.example.org" does not exist.
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And if "2.example.org" does not exist, there is also no direct match
for "x.2.example.org". The last step is to deny the existence of the
source of synthesis, to prove that no wildcard expansion was
possible.
The resolver hashes "*.example.org" to
"22670trplhsr72pqqmedltg1kdqeolb7" and checks that it is covered: in
this case by the last NSEC3 (see Figure 9), the hash falls in the
interval set by "1avvqn74sg75ukfvf25dgcethgq638ek" and
"75b9id679qqov6ldfhd8ocshsssb6jvq". This means there is no wildcard
record directly below the closest encloser and "x.2.example.org"
definitely does not exist.
When we have validated the signatures, we reached our goal:
authenticated denial of existence.
5.6. Three To Tango
One extra NSEC3 record plus additional signature may seem a lot just
to deny the existence of the wildcard record, but we cannot leave it
out. If the standard would not mandate the closest encloser NSEC3
record, but instead required two NSEC3 records: one to deny the query
name and one to deny the wildcard record. An attacker could fool the
resolver that the source of synthesis does not exist, while it in
fact does.
Suppose the wildcard record does exist, so our unsigned zone looks
like this:
example.org. SOA ( ... )
example.org. NS a.example.org.
*.example.org. TXT "wildcard record"
1.h.example.org. TXT "1.h record"
3.3.example.org. TXT "3.3 record"
The query "x.2.example.org TXT" should now be answered with:
x.2.example.org. TXT "wildcard record"
An attacker can deny this wildcard expansion by calculating the hash
for the wildcard name "*.2.example.org" and searching for an NSEC3
record that covers that hash. The hash of "*.2.example.org" is
"fbq73bfkjlrkdoqs27k5qf81aqqd7hho". Looking through the NSEC3
records in our zone we see that the NSEC3 record of "3.3" covers this
hash:
8555t7qegau7pjtksnbchg4td2m0jnpj.example.org. (
NSEC3 1 0 2 DEAD 15BG9L6359F5CH23E34DDUA6N1RIHL9H TXT RRSIG )
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This record also covers the query name "x.2.example.org"
("ndtu6dste50pr4a1f2qvr1v31g00i2i1").
Now an attacker adds this NSEC3 record to the AUTHORITY section of
the reply to deny both "x.2.example.org" and any wildcard expansion.
The net result is that the resolver determines that "x.2.example.org"
does not exist, while in fact it should have been synthesized via
wildcard expansion. With the NSEC3 matching the closest encloser
"example.org", the resolver can be sure that the wildcard expansion
should occur at "*.example.org" and nowhere else.
Coming back to the original question: why do we need up to three
NSEC3 records to deny a requested name? The resolver needs to be
explicitly told what the "closest encloser" is and this takes up a
full NSEC3 record. Then, the next closer name needs to be covered in
an NSEC3 record, and finally an NSEC3 must say something about
whether wildcard expansion was possible. That makes three to tango.
6. Security Considerations
DNSSEC does not protect against denial of service attacks, nor does
it provide confidentiality. For more general security considerations
related to DNSSEC, please see RFC 4033, RFC 4034, RFC 4035 and RFC5155 ([RFC4033], [RFC4034], [RFC4035] and [RFC5155]).
These RFCs are concise about why certain design choices have been
made in the area of authenticated denial of existence.
Implementations that do not correctly handle this aspect of DNSSEC,
create a severe hole in the security DNSSEC adds. This is
specifically troublesome for secure delegations: If an attacker is
able to deny the existence of a DS record, the resolver cannot
establish a chain of trust, and the resolver has to fall back to
insecure DNS for the remainder of the query resolution.
This document aims to fill this "documentation gap" and provide
would-be implementors and other interested parties with enough
background knowledge to better understand authenticated denial of
existence.
7. IANA Considerations
This document has no actions for IANA.
8. Acknowledgments
This document would not be possible without the help of Ed Lewis, Roy
Arends, Wouter Wijngaards, Olaf Kolkman, Carsten Strotmann, Jan-Piet
Mens, Peter van Dijk, Marco Davids, Esther Makaay, Antoin Verschuren,
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Internet-Draft Authenticated Denial in DNS February 2014Appendix A. On-line Signing: Minimally Covering NSEC Records
An NSEC record lists the next existing name in a zone, and thus makes
it trivial to retrieve all the names from the zone. This can also be
done with NSEC3, but an adversary will then retrieve all the hashed
names. With DNSSEC on-line signing, zone walking can be prevented by
faking the next owner name.
To prevent retrieval of the next owner name with NSEC, a different,
non-existing (according to the existence rules in []#RFC4592,
Section 2.2) name is used. However, not just any name can be used
because a validator may make assumptions on the size of the span the
NSEC record covers. The span must be large enough to cover the
QNAME, but not too large that it covers existing names.
[RFC4470] introduces a scheme for generating minimally covering NSEC
records. These records use a next owner name that is lexically
closer to the NSEC owner name than the actual next owner name,
ensuring that no existing names are covered. The next owner name can
be derived from the QNAME with the use of so-called epsilon
functions.
For example, to deny the existence of "b.example.org" in the zone
from Section 3.2, the following NSEC record could have been
generated:
a.example.org. NSEC c.example.org. RRSIG NSEC
This record also proves that "b.example.org" also does not exist, but
an adversary _cannot_ use the next owner name in a zone walking
attack. Note the type bitmap only has the RRSIG and NSEC set,
because [RFC4470] states:
The generated NSEC record's type bitmap MUST have the RRSIG and
NSEC bits set and SHOULD NOT have any other bits set.
This is because the NSEC records may appear at names that did not
exist before the zone was signed. In this case however,
"a.example.org" exists with other RR types and we could have also set
the A and TXT types in the bitmap.
Because DNS ordering is very strict, the span should be shortened to
a minimum. In order to do so, the last character in the leftmost
label of the NSEC owner name needs to be decremented and the label
must be filled with octets of value 255 until the label length
reaches the maximum of 63 octets. The next owner name is the QNAME
with a leading label with a single null octet added. This gives the
following minimally covering record for "b.example.org":
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a\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255.example.org. (
NSEC \000.b.example.org. RRSIG NSEC )
Appendix B. On-line Signing: NSEC3 White Lies
The same principle of minimally covering spans can be applied to
NSEC3 records. This mechanism has been dubbed "NSEC3 White Lies"
when it was implemented in Phreebird [phreebird]. Here, the NSEC3
owner name is the hash of the QNAME minus one and the next owner name
is the hash of the QNAME plus one.
The following NSEC3 white lie denies "b.example.org" (recall this
hashes to "iuu8l5lmt76jeltp0bir3tmg4u3uu8e7"):
iuu8l5lmt76jeltp0bir3tmg4u3uu8e6.example.org. (
NSEC3 1 0 2 DEAD IUU815LMT76JELTP0BIR3TMG4U3UU8E8 )
The type bitmap is empty in this case. If the hash of
"b.example.org" - 1 is a collision with an existing name, the bitmap
should have been filled with the RR types that exist at that name.
This record actually denies the existence of the next closer name
(which is conveniently "b.example.org"). Of course the NSEC3 records
to match the closest encloser and the one to deny the wildcard are
still required. These can be generated too:
# Matching `example.org`: `15bg9l6359f5ch23e34ddua6n1rihl9h`
15bg9l6359f5ch23e34ddua6n1rihl9h.example.org. (
NSEC3 1 0 2 DEAD 15BG9L6359F5CH23E34DDUA6N1RIHL9I NS SOA RRSIG
DNSKEY NSEC3PARAM )
# Covering `*.example.org`: `22670trplhsr72pqqmedltg1kdqeolb7`
22670trplhsr72pqqmedltg1kdqeolb6.example.org.(
NSEC3 1 0 2 DEAD 22670TRPLHSR72PQQMEDLTG1KDQEOLB8 )
Appendix C. List of Hashed Owner Names
The following owner names are used in this document. The origin for
these names is "example.org".
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